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| United States Patent |
5,257,405
|
|
Reitberger
|
October 26, 1993
|
Method and system for setting up LOS-radio communication between mobile
or stationary remote stations
Abstract
For setting up LOS radio links between mobile calling
transmitting/receiving stations and other mobile or stationary remote
transmitting/receiving stations in a predetermined operating area,
especially in the frequency range above 1 GHz, every mobile station is
provided with an electronic memory with an associated processor. The
memory stores the respective geographical and topographical data of the
operating area. Prior to the setting-up of a radio link, the transmission
loss between the stations is calculated in accordance with known model
calculations on the basis of the geographical and topographical data
stored in the memory by inputting into the processor the current location
data of the calling station and the remote station.
| Inventors:
|
Reitberger; Peter (Munich, DE)
|
| Assignee:
|
Rohde & Schwarz GmbH & Co. KG (DE)
|
| Appl. No.:
|
706256 |
| Filed:
|
May 28, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
455/514; 343/754; 455/67.13; 455/129; 455/289; 455/513 |
| Intern'l Class: |
H04B 001/18; H04B 017/02; H04B 007/005 |
| Field of Search: |
343/754
342/62,63,64
;226.2;226.3;278.1;280;283;289
455/33.1,33.2,33.3,33.4,52.1,54.1,54.2,56.1,129,62,63,67.1,67.3,67.7,254,226.1
|
References Cited
U.S. Patent Documents
| 4670906 | Jun., 1987 | Thro | 455/56.
|
| 4907290 | Mar., 1990 | Crompton | 455/56.
|
| 5046130 | Sep., 1991 | Hall et al. | 455/54.
|
| 5117503 | May., 1992 | Olson | 455/33.
|
| 5134709 | Jul., 1992 | Bi et al. | 455/33.
|
| Foreign Patent Documents |
| 0367935 | May., 1990 | EP.
| |
| 3012484 | Jun., 1982 | DE.
| |
| 3335128 | Apr., 1985 | DE.
| |
| 3417233 | Nov., 1985 | DE.
| |
| 3441722 | May., 1986 | DE.
| |
| 3337648 | Feb., 1987 | DE.
| |
Other References
VHF and UHF Propagation Curves for Land Mobile Services, Recommendation
529, Report 567.3 "Methods and Statistics for Estimating Field-Strength
Values in the Land Mobile Services Using the Frequency Range 30 MHz to 1
GHz", pp. 298-311.
"Automobile Telephone System", Japanese Patent Abstract, No. 60-158737
dated Aug. 20, 1985.
"Antenna Monitor Device", Japanese Patent Abstract, No. 60-150331 dated
Aug. 8, 1985.
CCIR Recommendation 529, 1978; "Methods and Statistics for Estimating
Field-Strength Values in the Land Mobile Services Using the Frequency
Range 30 MHz to 1 GHz", pp. 298-311.
39th IEEE Vehicular Technology Conference No. 89, May 1989, "Propagation
Considerations of Low Power Cellular Boosters and Case Histories", by
Ronald J. Jakubowski, pp. 523-527.
The ARRL Antenna Book, 1988, pp. 23-4-23-11.
|
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Charouel; Lisa
Claims
I claim as my invention:
1. A method for setting up an LOS radio link between at least one mobile
calling transmitting/receiving station and at least one remote
transmitting/receiving station, both located in a predetermined operating
area, comprising the steps of:
providing the mobile station with an electronic memory and an associated
processor;
storing at least geographical or topographical data of the operating area
in the memory at the mobile station;
prior to establishing the radio link, based on a current location of the
mobile calling station and location of the remote station, calculating a
transmission loss between the calling station and the remote station based
on at least the geographical or topographical data stored in the memory at
the mobile station by inputting into the processor at least the current
location of the mobile calling station; and
adjusting operation characteristics of the mobile station in accordance
with the calculated transmission loss.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a system for setting up so-called LOS
(line of sight) radio links between mobile transmitting/receiving stations
and other mobile or stationary transmitting/receiving stations with a
predetermined operating area, and preferably in a frequency band above 1
GHz.
2. Description of the Prior Art
In the case of LOS radio links of the kind used partly in the frequency
band above 30 MHz and predominantly in the frequency band above 1 GHz,
there exist severe limitations for use in mobile transmitting/receiving
stations. This is true since, with a change of location of the mobile
stations, the respective field strength attenuation or transmission loss
between the calling station and another mobile or stationary remote
station is unknown.
To prevent overloading of a radiotelephone network, it has been known to
continually determine the number of mobile radiotelephones present in a
radio cell, while the association of the radiotelephone to neighboring
radio cells is also determined in accordance with the geographical and
topographical conditions of the respective sub-area (DE 3,441,722). In
this connection, it has also been known to monitor continually the
transmission performance of the radio link and to take it into account for
control of a dynamic cell size.
For mobile radio links, it has also been known to determine the radio zone
boundary between two neighboring radio zones by measuring the relative
distance between a mobile subscriber and fixed stations (radio
concentrators) (DE 3,335,128). Also, a radio system has been known in
which the intercommunicating radio stations adjust the transmitter power
of the respective other station in response to the transmission
performance (DE 3,417,233). For a radio network comprising a plurality of
mobile stations it has also been known to have each station act as a relay
between mobile stations communicating in pairs (DE 3,337,648). Finally, it
has been known with mobile radio links to perform field strength
measurements so as to obtain a criterion for switching a mobile subscriber
from one radio area to the next (DE 3,012,484).
These known techniques are unsuitable for setting up optimum LOS radio
links and for solving the problems arising in this connection.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a simple system which
allows the setting-up of optimum LOS radio links also by mobile
transmitting/receiving stations in a frequency range above 1 GHz.
According to the method and system of the invention, an LOS radio link is
set up between a mobile calling transmitting/receiving station and another
mobile or stationary remote transmitting/receiving station in a
predetermined operating area. Each mobile station is provided with an
electronic memory and an associated processor. Respective geographical
and/or topographical data of the operating area of the mobile station is
stored in the memory. Prior to establishing a radio link, a transmission
loss is calculated between the calling station and the remote station on
the basis of the geographical and/or topographical data stored in the
memory by inputting into the processor current location data of the
calling station and possibly also the remote station, if not previously
entered into the memory.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a mobile calling station and a mobile or fixed remote station
and wherein a processor and memory containing topographical and/or
geographical data is stored according to the system and method of the
invention;
FIGS. 2-6 are graphs showing field strengths versus distance;
FIG. 7 is a graph showing variation of height with frequency; and
FIG. 8 is a graph showing deviation from median field strength curve due to
buildings surrounding mobile station.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With the system according to the present invention as shown in the drawing
figure, the respective transmission loss to the remote station 14 with
antenna 15 is calculated in every single mobile transmitting/receiving
station 10 for the current location on the basis of the geographical and
topographical data 16, 17 of the operating area 20 stored in said station
10. The optimum radio availability may then be set directly by
correspondingly changing the transmitting and/or receiving parameters of
the mobile calling station 10 based on the results of a calculation. This
may be done, for example, by a corresponding change of antenna 13 height
or a change of the directional pattern of the antenna 13, or a
corresponding change of the transmitter power or the receiver sensitivity
of the transmitter/receiver 10A, or even by a change of frequency, with
due consideration of the frequency response of the receiver. Of course,
such a change of the transmitter and/or receiver 10 parameters may also be
performed automatically in dependence upon the calculated transmission
loss by corresponding control means in the mobile calling station 10.
Another possibility of improving the radio availability resides in
changing the location of the mobile station until the desired optimum
transmitting/receiving conditions are achieved. To achieve this, it has
been found to be advantageous when the radio availability of neighboring
locations of the calling station is calculated and indicated at the same
time so that the user of the mobile station may immediately take up the
best possible location.
"Geographical data of the operating area" are data of the kind indicated in
maps, for example absolute altitude above sea level. "Topographical data"
refer to the terrain configuration of the operating area 20, to terrain
elevations, valleys etc. (surface structure), while "morphological data"
are details of buildings 17, of forested terrain, or upgrowth of the
operating area 20. All of these data exist already for various countries
and areas, and are stored in digital form in data memories so that they
can easily be utilized for the purposes of the present invention. Also,
they may be continually supplemented during operation in a simple way, for
example by having the mobile calling station continually shoot-such as by
means of video cameras, radar devices or the like-the topography and
morphology of the operating area during the station's movement through the
area, and perform a comparison with and possibly updating of the already
stored data, for example when a high building has in the meantime been
erected in the operating area. Such changes of, or supplements to, the
topography and morphology as determined by continuous observation are then
directly written into the electronic memory 2 via input/output device 18
and processor 11, and may be taken into account when the radio link is
established.
The calculation of the field strength attenuation, i.e. the radio
availability between a transmitting and a receiving station with due
consideration of the geographical and topographical data of an operating
area, is known per se for the planning of large-area radio links (CCIR
Report 567-3). Based on an empirical formula (formula derived by Okumura)
the basic transmission loss can be calculated as dependent upon frequency,
distance, mobile station effective antenna height and remote station
antenna height (calculation with reference to geographical data).
Moreover, in accordance with these known calculation models, the
transmission loss calculated with reference to geographical data may be
corrected by factors related to the density of buildings and of vegetation
in the vicinity of the station (consideration of topographical and
morphological data). This known model calculation is performed directly in
every mobile station by use of data stored in the memory 12 which is
controlled and processed by use of the input/output device 18 such as a
keyboard and the processor 11, with due consideration of all stored
geographical, topographical, and morphological data of the operating area
for the respective current location of the calling station and the remote
station. Thus, the field strength attenuation between the calling station
and the remote station is directly determined and may then be taken into
account or changed correspondingly when the radio link is actually set up.
As mentioned previously, the calculation of the field strength attenuation
is known from CCIR report 567-3, and the Okamura formula. Hereafter, that
report is now set forth.
1. Introduction
Propagation in the land mobile services at frequencies in the 30 MHz to 1
GHz range is affected in varying degrees by topography, vegetation,
man-made structures, ground constants, the troposphere and the ionosphere.
Curves are provided hereafter for predicting field strength under average
conditions for three frequency ranges. Analytical expressions are also
provided which are valid for certain frequency ranges and conditions, and
various correction factors which can be used to refine the average
predictions. Experimental results submitted by individual administrations
are described hereafter.
The material herein is statistical in nature and oriented towards
application to planning and system design.
2. Propagation Curves
FIGS. 2 and 3 show curves for 450 MHz and 900 MHz at mobile antenna heights
of 1.5 m, base station heights between 30 and 1000 m, 50% of the locations
and 50% of the time. These particular curves were derived from
measurements made in urban areas of Japan (Okumura et al), and should be
compared with data from other areas where available.
Measurements made in Japan have shown that the height gain factor from 1.5
to 3 m is 3 dB in urban areas for the UHF band. This value can be used to
estimate the median field strength for mobile antenna heights of 3 m using
the data for Bands IV and V (Band IV:470-582 MHz; Band V:582-960 MHz) for
urban areas as given in FIGS. 2 and 3.
Based on the work of Okumura et al, Table 1 provides an empirical formula
for calculating basic transmission loss for distances up to 20 km relating
to FIGS. 2 and 3.
TABLE I
______________________________________
Empirical formula for basic transmission loss
______________________________________
L.sub.b = 69.55 + 26.16 log .function. - 13.82 log h.sub.1 - a(h.sub.2)
(44.9 - 6.55 log h.sub.1) log R dB
correction factor for h.sub.2 :
a(h.sub.2) = (1.1 log .function. - 0.7)h.sub.2 - (1.56 log .function. -
0.8)
Where:
.function.:
frequency 450-1000 MHz
h.sub.1 :
base station effective antenna height
30-200 m
h.sub.2 :
vehicular station antenna height
1-10 m
R: distance 1-20 km
______________________________________
Theoretical curves for the VHF band were also derived by Okumura et al for
the urban environment. The empirical formula given in Table 1 may be
applicable for frequencies in the range 150-1 500 MHz, and field-strength
measurements at a frequency of 160 MHz, made in an urban area of Madrid,
Spain, were found to be in good agreement with this model. Further data
are required to investigate the validity of the model in terms of
frequency range and receiving antenna height.
FIG. 8 provides a correction factor for use with FIG. 1 where the density
of buildings in the vicinity of the mobile station is greater or less than
the reference density for which the curves were derived (15% of the area
covered by buildings).
FIGS. 4, 5, and 6 show propagation curves which are valid for frequencies
approximately between 100 and 250 MHz, mobile antenna heights of 3 m, base
antenna heights between 10 m and 600 m, for rural conditions, for 50% of
the locations, and for 50%, 10% and 1% of the time.
The curves in FIGS. 4,5, and 6 were derived from corresponding curves in
CCIR with appropriate corrections for a mobile station antenna height of 3
m. A correction of 8 dB was applied for distances up to 50 km and 4 dB for
distances greater that 100 km with linear interpolation for intermediate
distances.
The mobile station height correction factors used here for rural areas were
based on work described in various publications and work done in the
United Kingdom. However, more recent work in the United Kingdom, the USSR,
the Federal Republic of Germany, Switzerland and the United States of
America suggests that height gain factors may not be distance dependent
but may be dependent on terrain irregularity and objects in the vicinity
of the mobile station.
The curves in FIGS. 4, 5, and 6 for base station antenna heights of 20 and
10 m were derived from the 37.5 m curves by applying distance dependent
correction factors which were based on a theoretical study by the Federal
Republic of Germany.
Generally, the effective antenna height of the base station intended to be
used with FIGS. 2 to 6 herein and with the formulae in Table 1 is defined
as the height of the antenna over the average level of the ground between
distances of 3 and 15 km from the base station in the direction of the
mobile station.
Under some conditions, particularly for short distances of only a few
kilometers or if the mobile station is situated higher than the base
station, the definition for base station antenna height given above may
lead to arithmetic results without physical significance. A study in the
Federal Republic of Germany found that the following definition led to
better results on the average:
##EQU1##
where: h.sub.1 :base station effective antenna height,
h.sub.b :antenna height above ground at the base station,
h.sub.Ob :terrain height above sea level at the base station,
h.sub.Om :terrain height above sea level-the mobile station.
Field-strength measurements carried out in rural areas in the People's
Republic of Poland at frequencies between 34 and 306 MHz have shown good
agreement with the propagation curves of Recommendation 370 when allowance
is made for terrain effects.
Measurements carried out in the Canadian arctic at frequencies of 148 and
450 MHz at distances up to 100 km suggest that in such regions field
strengths may be greater than those predicted by FIG. 4.
Measurements carried out in the People's Republic of Poland over a
trans-horizon path on a frequency of 342 MHz have shown that the 1% values
of field strength exceed the values estimated on the CCIR curves by 2 to 8
dB and that the 1% and 10% values show marked variations between day and
night.
The relationship between the field strength, E'(Db(.mu.V/m), for 1 kW
radiated from a halfwave dipole) as found from the curves herein and the
basic transmission loss, L.sub.b, (i.e., the loss between isotropic
antennas) is given by:
L.sub.b =139.4+20 log .function.MHz-E'dB (1)
The transmission loss between halfwave dipoles, L.sub.d is given by:
L.sub.d =135.1+20 log .function.MHz-E'dB (2)
Measurements made in a multipath situation may not reflect the
relationships shown in equations (1) and (2) above.
Field strength data taken in several metropolitan areas in the United
States at 900 MHz (see .sctn.5.2 below) generally show good agreement with
the distance trends of the curves for urban areas shown in FIGS. 2 and 3.
However, some of the surveys showed significant difference of average
field-strength levels from these curves.
Results of comparisons at 450 MHz of measured path losses around London
with those predicted from computer-based procedures incorporating terrain
height information from a United Kingdom terrain data bank have been
previously reported. Separate measurements were conducted along radial,
circumferential and mixed-path routes.
3. Calculation of Field-strength Values
For line-of-sight paths for frequencies at VHF and slightly lower the field
strengths may be calculated using the method described below which is
based on work done in the People's Republic of Poland following work done
in the United States. The method is appropriate for unobstructed
propagation paths since no account is taken of the effects of the local
environment, e.g. buildings.
The use of an effective antenna height based on the electrical
characteristics of the ground is especially applicable at the lower
frequencies, for lower antenna heights, for vertical polarization, and
over wet ground.
A first order estimate of the median field-strength E(.mu.V/m) is given by:
##EQU2##
p.sub.1 :effective radiated power of the transmitter, using a half-wave
dipole antenna (W),
d:distance between antennas (km),
h.sub.1 :effective height of the transmitting antenna (m),
h.sub.r : effective height of the receiving antenna (m),
.lambda.:wavelength (m).
These effective heights are given by:
##EQU3##
where: h.sub.1 :actual height of the transmitting antenna (m),
h.sub.2 :actual height of the receiving antenna (m), and
h.sub.0 :(m) is obtained.
##EQU4##
where: .lambda.:wavelength (m),
.epsilon..sub.r :relative permittivity,
.sigma.:conductivity of the ground (S/m).
For horizontal polarization, at frequencies above approximately 40 MHz, the
effective heights of the transmitting and receiving antennas may be
assumed to equal the actual heights. To aid calculation when using
vertical polarization, FIG. 7 is a graphical representation of equation
(6) for various types of terrain.
Equation (3) is valid within the region where the field decreases
monotonically with distance up to the radio horizon.
Corrections for terrain topography, vegetation and man-made structures may
be added to equation (3), and are discussed in CCIR Recommendation 370 and
CCIR Report 239.
At distances beyond the radio horizon at frequencies below 90 MHz and for
small percentages of time, the effects of the ionosphere may be important.
4. Depolarization Phenomena
The depolarization factor is defined as the ratio of the amplitude of the
orthogonally polarized component, produced by some propagation mechanism,
to the amplitude of the original plane polarized wave. For land mobile
systems it may be sometimes more convenient to consider the polarization
discrimination factor is normally expressed in decibels and is, in
practice, of the opposite sign but numerically equal to the depolarization
factor provided that the latter is not too small.
Measurements in Sweden of the depolarization effect, with both antennas at
a low height (less than 10 m), have shown that the depolarization factor
increases with increasing frequency from about -18 dB at 35 MHz to about
-7 dB at 950 MHz.
The depolarization factor is log-normally distributed with a standard
deviation somewhat dependent on the frequency. The average value of the
difference between the 10% and 90% values (in the frequency range 30 to
1000 MHz) is about 15 dB. Whether the original polarization is vertical or
horizontal has been observed to make only a slight difference in this
respect.
Two types of time variation of the depolarization effect have been found.
The first is a slow variation resulting from the changing electrical
properties of the ground with weather conditions. This effect is most
pronounced at lower frequencies. The second is due to the motion of trees
which gives a depolarization fading phenomenon amounting to several
decibels in amplitude at quite moderate wind velocities.
5. Attenuation Due to Vegetation and Buildings
Signals transmitted to and from moving vehicles in urban or forested
environments exhibit extreme variations in amplitude due to multiple
scattering. Fades of 30 dB or more below the mean level are common. A
number of investigators have reported that the instantaneous field
strength when measured over distances of a few tens of wavelengths is
approximately Rayleigh-distributed. The mean values of these small sector
distributions vary widely from area to area, depending on height, density
and distribution of trees, buildings and other structures. Studies
concerning these variations are described below. Some studies concerning
signal attenuation through the walls of buildings are also described.
5.1 Attenuation Due to Vegetation
A general discussion of signal attenuation in forested terrain appears in
CCIR Report 236 which describes the various paths and mechanisms for
propagation through an idealized forest environment and presents models
giving attenuation as a function of frequency and length of path through
the trees. These models cover situations where both antennas are within
the forest and where at least one antenna is within or close to the edge
of the trees. In the case where both antennas are well clear of a grove of
trees, the path can be treated as a diffraction path.
CCIR Report 239 discusses the case where both transmitter and receiver are
located above the forest as in the broadcast and radio-relay systems. For
mobile systems where the base station antenna is located outside or above
the trees and mobile stations move in and out of wooded areas, a method
described by Kinase may be useful. This method determines attenuation from
median field-strength curves as a function of frequency and percentage of
an area covered by clutter, including both trees and buildings.
Measurements were carried out in Washington D.C. to determine the variation
in received signal levels due to changes of season for mobile stations
operating in woodland areas. Cumulative statistics for the relative signal
levels with trees in full leaf and without leaf were determined for
transmission at 459 and 955 MHz. The median seasonal difference for both
frequencies was found to be approximately 4.5 dB and the upper decile was
approximately 6 dB.
5.2 Attenuation Due to Buildings
According to experimental results in Japan, the median field strength is
particularly affected by the buildings around a mobile station, because
the mobile station antenna height is almost always lower than these
buildings. At 450 MHz these buildings cause a considerable deviation of
the median field strength in any small area (of about 0.25 km.sup.2)
compared with the reference median field strength typical of urban areas,
as shown in FIG. 2. This local deviation from the reference value is shown
in FIG. 8 as a function of the parameter .alpha., where .alpha. is defined
as the percentage of the area covered by buildings. Each point in the
graph is the deviation of the median field strength for one such small
area. The values of median field strength in the 800 MHz band were found
to be strongly correlated with those in the 400 MHz band. The regression
line in FIG. 8 for .alpha. less than 5% was determined from data measured
at 800 MHz in Japan which included locations with values of .alpha. less
than 1%.
Measurements in urban areas in the People's Republic of Poland at
frequencies between 34 and 306 MHz show that the spread of field-strength
values seems to be almost independent of frequency.
Field strength measurements at 900 MHz in several metropolitan areas in the
United States show significant differences in average signal level from
city to city not accounted for by differences in terrain. Variations in
small-area (of the order of 0.25 km.sup.2) medians about the average level
at a given distance show standard deviations of 3 to 12 dB. An attempt is
being made to relate these variations in signal level and standard
deviation to the distribution of trees and buildings.
5.3 Building Penetration Loss
The attenuation of radio waves through the walls of buildings is a crucial
factor in the feasibility and design of portable radio communications and
paging systems. Studies of this attenuation are reported below.
Measurements at 940 MHz were carried out in a medium-size city in the
United States (Louisville, Ky.) to determine building penetration losses
for hand-held portable radios. These measurements were averaged for 3.7 by
3.7 m areas so as to average the effects of multipath fading. For typical
steel and concrete and stone office buildings these averages were found to
be normally distributed between 10 and 90% with a mean of 10 dB and a
standard deviation of 7.3 dB. These statistics may be used with the
field-strength values determined from FIG. 2 to estimate probable
field-strength levels on the ground floor of office buildings comparable
in nature to those in the test city.
Measurements were carried out on a number of residential suburban houses in
the United States to determine building penetration losses. Medians of the
envelope variations over small areas (1.2 m by 1.2 m) were used to
determine attenuation statistics for the houses. Cumulative distributions
for building penetration losses for the ground and first floors were
approximately log-normal with median values of 5.8 dB and 0.1 dB, and
standard deviations of 8.7 dB and 9.0 dB, respectively.
Measurement of building penetration losses at 850 MHz were carried out on
14 office and industrial buildings in a large city in the United States.
Ground floor penetration losses averaged 18.0 dB with a standard deviation
of 7.7 dB for buildings in the urban area and 13.1 dB with a standard
deviation of 9.5 dB for buildings in the suburban area. The overall
decrease of penetration loss with height was about 1.9 dB per floor. For
individual buildings the losses on upper floors were influenced by the
height of surrounding buildings. The average loss for areas with windows
was about 6 dB less than that for areas without windows.
In order to characterize signal statistics for a severe case, attenuation
measurements were carried out in a steel sheel building at 900 MHz in the
United States. Cumulative statistics of relative signal strengths inside
and outside the building for receiver antennas located at heights of 1 to
2 m above the floor were approximately Rayleigh-distributed with a median
attenuation of 28.5 dB.
Building penetration loss in the 900 MHz band was measured in several kinds
of buildings in Japan. The measured penetration losses from the entrance
to the core of the buildings along the corridors ranged from 1 to 2 dB/m.
5.4 Body Effect Loss
In the design of portable telephone and radio paging systems, it is
important to quantify the loss due to the presence of a body in the
multipath field. The degradation in effective antenna performance due to
scattering and absorption by a human body was measured in the 900 MHz band
in an urban area. When the dipole antenna was placed at the waist and
shoulder of a human, the received field strength decreased by 4-7 dB and
1-2 dB respectively, in comparison with values of received field strength
using an antenna held several wavelengths away from the body.
6. Dependence of Field Strength on Time, Location and Nature of Terrain
CCIR Recommendation 370 and CCIR Report 239 consider problems of the
dependence of field strength on time location and the nature of the
terrain.
The parameter .DELTA.h is used to define the degree of terrain irregularity
(see CCIR Recommendation 310) and in this case, it is determined for the
range 10 km to 50 km from the transmitter. Its derivation and use in
correcting median field-strength values for areas where the terrain is
smoother or rougher than average is described in CCIR Report 239. The
range of variation of the log-normal location distribution, i.e., the
standard deviation, .sigma..sub.L, increases with increasing values of
.DELTA.h and frequency.
For many practical purposes, for instance the evaluation of the probability
of interference when preparing frequency plans for mobile radio, the
following approximations have been shown to be useful and sufficiently
accurate:
the time and location distributions of field strength are assumed to be
log-normal (Gaussian (dB)) in the range of interest (between about 5% and
50% for the time distribution);
the standard deviations of the location-and time-distributions
(.sigma..sub.L and .sigma..sub.1) are derived from CCIR Recommendation
370, assuming a Gaussian time distribution for time percentages between 5%
and 50% (see Table II);
the combined standard deviation is given by .sigma.=.sigma..sub.L.sup.2
+.sigma..sub.12.
TABLE II
__________________________________________________________________________
Standard deviations .sigma..sub.L and .sigma..sub.1
Band
.sigma..sub.L (dB) .sigma..sub.1 (dB)
__________________________________________________________________________
VHF
8 d(km) 50 100 150 175
Land and Sea
3 7 9 11
UHF
.DELTA.h(m)
50 150
300 Land 2 5 7
10 15
18 Sea 9 14 20
__________________________________________________________________________
Note -- The values in Table II refer to rural areas. They should be
applied with caution when used with FIGS. 1 and 2, which are for urban
areas.
A recent analysis of a large amount of measurement data in the United
States led to the derivation of the following expression relating location
variability (standard deviation), .sigma..sub.L, to the transmission
wavelength, .lambda., and terrain irregularity, .DELTA.h, for
.DELTA.h/.lambda..ltoreq.3000:
.sigma..sub.L =6+0.69(.DELTA.h/.lambda.).sup.1/2
-0.0063(.DELTA.h/.lambda.)dB (8)
For (.DELTA.h/.lambda.)>3000,.sigma..sub.L =25dB.
The coefficients in equation (8) differ from those in the original equation
in [Longley, 1976], where .DELTA.h is defined as a function of distance
rather than for a fixed range of 10 to 50 km.
The data used in this study were spot measurements obtained over paths
ranging from 0.5 to 120 km in length, with antenna heights from 0.6 to 15
m, over terrain ranging from plains to rugged mountains and at frequencies
from 30 MHz to 10 GHz.
In interference calculations it is sometimes desirable to take into
consideration the correlation between signals received at the mobile
station from non co-sited transmitters. A study of data at 913 MHz in
suburban to urban surroundings in the United States of America found some
positive correlation between signals received at a mobile station from
base station sites separated by as much as 55.degree. in azimuth.
Correlation between signals transmitted over paths with a common
transmitting or receiving terminal is also discussed in CCIR Report 228,
based on measurements at VHF.
6.1 Local Variations in Field Strength
Local variations over distances of several meters, due to multipath
propagation, will often be significant in built-up areas and near trees.
The variations will also depend on polarization. For example, 3 or 4-story
buildings might cause an average diffraction or screening loss of 10-12 dB
at VHF for an antenna a few meters above ground in the adjacent street;
however, the local variations in that street would be of the order of 5 dB
(peak-to-peak) with horizontal polarization and 10-11 dB with vertical
polarization. Local variations at VHF in a tree-lined road (with the trees
the only substantial obstacles) are likely to be of the order of 4-5 dB
with horizontal polarization and 10-12 dB with vertical polarization. In
hilly country, the minimum field strength tends to occur on the near-side
slope of a valley rather than at its deepest point, and this minimum tends
to be lower with horizontal polarization than with vertical. On the side
of the hill facing the transmitter, horizontally polarized signals are
some 3-6 dB higher than for vertical polarization at frequencies around
100 MHz.
This concludes the CCIR Report 567-3.
The system in accordance with the present invention is particularly suited
for mobile radio systems in which a multiplicity of mobile stations 10
operate simultaneously in an operating area, for in that case the
respective calculated transmission losses may be interchanged between the
mobile stations 10 themselves and between the mobile stations and the
fixed stations 14 often provided in such systems. When a calling station
10 finds during setting up of a radio link that a direct link between it
and a remote station 14 with its antenna 15 (either mobile or stationary)
is too poor, for example, but that there is good communication with a
neighboring station which in turn has good communication with the desired
remote station, the link can be set up in known manner through the
neighboring station, which then acts as a relay station.
In accordance with a further improvement of the invention, it has been
found appropriate when, after a radio link has been established, the
actually occurring characteristic radio parameters such as transmitter
level, receiver level, phase jitter, S/N ratio, transmission loss etc. are
measured during the subsequent radio operation between the calling station
10 and the remote station 14, and are also stored in the electronic memory
12 via a processor 11and input/output device 18 for the respective
location of the mobile station, so that eventually a data bank will be
established in the memory 12 of every mobile station 10 which, in addition
to the geographical and topographical data of the operating area for
various locations, respectively represents the quality of the radio link
with selected remote stations. When a radio link is to be established, the
mobile calling station 10, after inputting its current position through an
input/output device 18, may immediately call up the optimum conditions for
a LOS radio link with a selected remote station, so that even a
calculation on the basis of the geographical data may become unnecessary.
The system according to the present invention allows optimum operation of a
mobile radio system, since, by direct change of the transmitting/receiving
parameters and possibly also the type of modulation of the transmitted
signals, quite specific radio link conditions in the operating area can be
achieved. Thus, it may be possible to choose the parameters on the basis
of the calculated data in such a way that direct communication between a
mobile station 10 and a remote station 74 exhibits optimum performance,
while the antenna height of station 74 does not, however, exceed a
predetermined height and the receiving level in station 70 is above a
predetermined threshold, so that interception of the link between station
10 and station 74 is no longer possible at a given distance.
Moreover, the system according to the present invention allows the
controlled setting-up of active and/or passive reflectors 19 at precisely
defined, and in most cases exposed, geographical locations in the
operating area to improve the radio link. Such additional reflectors will
either passively reflect the incident signals from the calling station or
will amplify the signals and re-transmit them in a given direction after
power amplification (frequency converter). Thus, the signals may also be
transmitted to an area which cannot be reached by direct LOS
communication. Such passive reflectors, for example, may be aluminum
strips (chaff) which are dispensed from the air over a certain area so
that an area is made available which previously was in the radio shadow of
a calling station. Such passive reflector layers may also be obtained by
ionization of certain air layers by chemical reactions in the atmosphere,
or by high-energy light stimulation (laser irradiation).
Although I have described my invention by reference to particular
illustrative embodiments thereof, many changes and modifications of the
invention may become apparent to those skilled in the art without
departing from the spirit and scope thereof. I therefore intend to include
within the patent warranted hereon all such changes and modifications as
may reasonably and properly be included within the scope of my
contribution to the art.
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